Math equation and numeric label - align environment

Question

I'm writing a large equation for which I need to introduce line breaks in order to make it more legible. I'm using the following code:

\documentclass{report}
\usepackage{mathtools}

\begin{document}
\begin{align}
        H &= \lambda_{R_x}V_x + \lambda_{R_y}V_y + \lambda_{R_z}V_z \nonumber \\
          &\mathrel{\phantom{=}} \kern+\nulldelimiterspace +\,\lambda_{V_x}\left(\frac{T}{m}\cos{\psi}\cos{\phi}\right) \nonumber \\ 
          &\mathrel{\phantom{=}} \kern+\nulldelimiterspace +\,\lambda_{V_y}\left(\frac{T}{m}\cos{\psi}\sin{\phi}\right) \nonumber \\
          &\mathrel{\phantom{=}} \kern+\nulldelimiterspace +\,\lambda_{V_z}\left(\frac{T}{m}\sin{\psi} - g\right) \nonumber \\
          &\mathrel{\phantom{=}} \kern+\nulldelimiterspace +\,\lambda_{m}\left(-\frac{T}{c}\right) \nonumber \\
          &\mathrel{\phantom{=}} \kern+\nulldelimiterspace +\,\mu_{u}\left(u^2_x + u^2_y + u^2_z -1\right) \nonumber\\
\end{align}

\end{document}

The output is the following:

I don't know what I should modify within the code in order to make the equation number aligned with the last math expression. As you can see, the equation number appears further down.

Answer 1

If you want to number only the last equation it is easier to use aligned with the [b] specifier, which spares you typing all the \nonumbers. Note also that your construction to align the + signs and get the correct spacing is overly complicated. You can put & after = and preserve the correct spacing by adding an empty group ={}&.

\documentclass{report}
\usepackage{mathtools}

\begin{document}

\begin{equation}
\begin{aligned}[b]
H ={}& \lambda_{R_x}V_x + \lambda_{R_y}V_y + \lambda_{R_z} V_z  \\
     &+\lambda_{V_x}\left(\frac{T}{m} \cos\psi \cos\phi\right) \\ 
     &+\lambda_{V_y}\left(\frac{T}{m} \cos\psi \sin\phi\right) \\
     &+\lambda_{V_z}\left(\frac{T}{m} \sin\psi - g\right) \\
     &+\lambda_{m}\left(-\frac{T}{c}\right) \\
     &+\mu_{u} \left(u^2_x + u^2_y + u^2_z -1\right)
\end{aligned}
\end{equation}

\end{document}

Personally I'd get rid of all \left/\right, and replace them by \biggl/\biggr. (Except the last one of course.) (But do you really need them?) Note also that

\cos{\psi}

isn't doing what you probably think, since \cos & Co. take no argument. If you don't want to write \cos(\psi), then \cos\psi is absolutely fine.

Answer 2

Here's another aligned-based solution, which gets by without having to produce large parentheses.

\documentclass{report}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}[b]
H &= \lambda_{R_x}V_x + \lambda_{R_y}V_y + \lambda_{R_z}V_z \\
  &\quad+\frac{T}{m}\bigl(\lambda_{V_x}\cos\psi\cos\phi
                         +\lambda_{V_y}\cos\psi\sin\phi  
                         +\lambda_{V_z}(\sin\psi - g) \bigr) \\
  &\quad-\frac{T}{c}\,\lambda_{m}  +\mu_{u}(u^2_x + u^2_y + u^2_z -1)
\end{aligned}
\end{equation}
\end{document}